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EZPZ ...

Digital ISO is like turning up the gain on an audio recorder. It’s definitely not the same as moving the mic closer to the source or making the source louder.

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No analogy is perfect, but this one holds up decently. If you over crank the gain in the recorder you get distortion. If you over crank your ISO you get noise. If your mic is far from the source and you increase gain (but short of distortion) you lose the sense of immediacy. If conditions call for higher ISO you lose dynamic range.

frankraney Loc: Clovis, Ca.

I think it is good, as long as it is explained. As most are saying, iso does not change the amount of light, but sensitivity to. Just as in the old days, it was the same for film. Iso is one side of the triangle, and we adjust the other two, ss and f-stop for the exposure we want. I look at it as, set the iso first (like putting the film in the camera), then adjusting the other two. The BIG advantage of digital is we can change iso (film) instantly for the conditions.

R.G. Loc: Scotland

That can be used as a starting point, which is what I'd do. "Results" in this case would mean the captured image before any post processing. Most of the following applies to both film and digital, but the focus is on digital, which will be the main interest of most people learning photography today.

Getting the exposure right:-

Too bright = overexposed. Too dark = underexposed.

There are two things that determine the exposure -

1) The amount of captured light.

2) The amount of amplification that the sensor signal is given in-camera (digital only).

The two factors that determine the amount of captured light are:-

1) The size of the aperture (i.e. the f-stop).

2) The duration of the exposure (i.e. the shutter speed).

- where a wider aperture (a lower f-stop) and a slower shutter speed both increase the amount of captured light.

Capturing more light results in a brighter exposure because it results in a stronger signal from the camera's sensor. However, that can be overdone because the sensor has limits which can be exceeded, resulting in what is referred to as "blown highlights" (a phenomenon also referred to as the sensor becoming saturated).

Increasing the ISO brightens the captured image, and that is achieved by electronically amplifying the sensor's signal. However, noise, which is inherent in the sensor's signal, is also amplified along with the signal, which is why the use of high ISOs (i.e. the use of high amplification) results in an increase in noise. The "noise" manifests as a graininess in the captured image, and in extreme cases it causes noticeable softness and a loss of colour accuracy.

Some cameras are more prone to high ISO noise than others. More recent cameras are generally better than older models, but it is also true that some camera brands are better than others when it comes to high ISO performance. Even within a single brand, the cameras are not all equal, but all cameras have an upper ISO limit beyond which the ISO noise will be seen as unacceptable.

At this point the explanation has already covered the basics of the exposure triangle and also covered the need to avoid blown highlights and high ISO noise. And that has been achieved with the minimum of "techno".

JD750 Loc: SoCal

Maybe it helps you relate to it and that’s ok. But No it is definitely not the same. Additional random noise is not the same as distortion introduced by clipping. Very different effects.

Gallopingphotog

This looks very useful for entry-level shooters. Myself I don't see the need for a big explanation of what ISO is/does at this point. It's liable to just confuse a newbie. They need to know that ISO affects light, and how it affects/is affected by the other two controls. But I think the technical stuff can wait a bit.

RCJets Loc: Virginia

Can you say that one step increase, or decrease of ISO, changes the sensitivity by a factor of two, either + or - as would one of the other two factors? In other words, would going from ISO 200 to ISO 400 would have about the same effect of going from f2.8 to f1.4?

anotherview Loc: California

Yep.

rmalarz Loc: Tempe, Arizona

As I stated earlier:
Exposure is simply the amount of light and the duration, aperture and shutter speed.
Photographic Exposure is the, above-referenced, amount of light falling on a photosensitive material. That's where ISO comes into play.
--Bob

R.G. Loc: Scotland

ISO is a linear scale, so if you want a doubling you literally have to double the number. For example, going from 200 to 400 is one stop (a doubling), and going from 400 to 800 is one stop.

F-stops are an exponential scale, so each step indicates a doubling. For example, going from f/2.8 to f/2.2 is a doubling (of the aperture) and going from f/2.2 to f/1.4 is a doubling. The f-stop steps are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 etc. As you can see, the steps aren't evenly sized, but they do each represent 1 stop. So going from f/1.4 to f/2.8 represents a jump of 2 stops.

Abo

Can you tell us RG, in the immortal words of Professor Julius Sumner Miller; "why is it so?" vis a vis
your aperture commentary.

BebuLamar

If two people sunbathing at the same location and at the same time. The two people receive the same exposure to sunlight although one of them has a serious sunburn and the other one is not. That doesn't mean the one with sunburn got more exposure to sunlight.

Bill_de Loc: US

Aperture and shutter speed affect the amount of light that hits the sensor. Changing the ISO will brighten or darken your image without affecting the amount of light that reaches the sensor. It's kind of like turning the volume up on a radio.

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JD750 Loc: SoCal

Yes in theory that is how it works. In practice you get more noise going from ISO 200 to ISO 400 (one stop). And if going from F2.8 to f1.4 (one stop) you get a decrease in depth of field. But both yield the same increase in exposure value (EV).

CamB Loc: Juneau, Alaska

I like this answer. Cuts to the chase simply. Teaching to those new to photography is all about simple and easy to understand. Techno comes later (if at all)
…Cam

JD750 Loc: SoCal

Because
f-stop = focal length / aperture_diameter

But light is proportional to the area of the aperture opening and

Area = pi/4 x diameter_squared